Pressure Levels and Terminology

Atmospheric Pressure—The atmosphere that surrounds the earth can be considered a reservoir of low-pressure air. Its weight exerts a pressure that varies with temperature, humidity, and altitude.

For thousands of years, air was considered weightless. This is understandable, since the net atmospheric pressure exerted on us is zero. The air in our lungs and the blood in our cardiovascular system has an outward pressure equal to (or perhaps slightly greater than) the inward pressure of the outside air. Since we feel no pressure, we are unaware of the air's weight.

The weight of the earth's atmosphere pressing on each unit of surface constitutes atmospheric pressure, which is 14.7 psi (101,300 Pa or 0.1013 MPa) at sea level. This pressure is called one atmosphere. In other commonly used units, one atmosphere equals 29.92 inches of mercury (in. Hg), 760 mm Hg (or 760 torr), and 1.013 bar(1 bar=0.1 MPa).

Since atmospheric pressure results from the weight of the overlying air, it is less at higher altitudes. As Fig. 3 shows, atmospheric pressure in Denver, Colorado (altitude 5,280 feet), is only 12.2psi. And in Mexico City, Mexico (altitude 7,800 feet), it is 11.1 psi. On top of Mount Everest, the pressure has fallen to one-third of an atmosphere.

Atmospheric pressure also varies from time to time at a single location, due to the movement of weather patterns. While these changes in barometric pressure are usually less than one-half inch of mercury, they need to be taken into account when precise measurements are required.

Gauge Pressure—Atmospheric pressure serves as a reference level for other types of pressure measurements. One of these is "gauge pressure."

Gauge pressure is either positive or negative, depending on its level above or below the atmospheric pressure reference. For example, an ordinary tire gauge showing 30 pounds (actually, 30 psi) is showing the excess pressure above
atmospheric. In other words, what the gauge shows is the difference between atmospheric pressure and the pressure of the air pumped into the tire. Gauge pressures can be either positive (above atmospheric) or negative (below atmospheric). Atmospheric pressure represents zero gauge pressure.

Absolute Pressure—A different reference level is used to obtain a value for "absolute pressure." This is pressure measured above a perfect vacuum. It is composed of the sum of the gauge pressure (positive or negative) and the atmospheric pressure. Where there might be confusion, gauge and absolute pressures are distinguished by adding the letter "g" or "a," respectively, to
the abbreviation for the units ("psig" or "psia").To obtain the'absolute pressure, simply add the value of atmospheric pressure (which averages 14.7 psi at sea level) to the gauge pressure reading. To find the current value of atmospheric
pressure in psia at a given location, multiply the barometer reading in in. Hg by 0.491. This conversion factor arises from the fact that a cube of mercury with one inch sides weighs 0.491 pound and thus exerts a pressure of 0.491 psi.

Using the simple tire pressure example, the absolute pressure including the atmospheric pressure—exerted by the air within the tire is 44.7 psia (30 psig plus 14.7 psi). Thus, the absolute pressure is 14.7 psi more than would be read on a tire-pressure gauge. Absolute pressure must be used in virtually all calculations involving pressure ratios.

Vacuum—Vacuum is a pressure lower than atmospheric. Except in outer space, vacuums occur only in closed systems.

In the simplest terms, any reduction in atmospheric pressure in a closed system may be called a partial vacuum. In effect, vacuum is the pressure differential produced by evacuating air from the system.

In a vacuum system more sophisticated than a suction cup, the enclosed space would be a valve actuator or some appropriate work device. A vacuum pump would be used to reduce atmospheric pressure in the closed space. The same principle would apply, however.

By removing air from one side of an air-tight barrier of some sort, atmospheric pressure can act against the other side. Just as with the suction cup, this action creates a pressure differential between the closed system and the open atmosphere. The pressure differential can be used to do work.

For example, in liquid packaging (bottling), reducing the pressure in a bottle (the enclosed space) makes the filling operation go much faster because the liquid or other material is literally pulled into the bottle, rather than simply falling by gravity.

Vacuum is usually divided into four levels:

Low vacuum represents pressures above one torr absolute. Flow in this range is viscous, as represented by most common fluids. Mechanical vacuum pumps are used for low vacuum, and represent the large majority of pumps in industrial practice.

Medium vacuum represents pressures between 1 and 10-3 torr absolute. This is a transition range between viscous and molecular flow. Most pumps serving this range are also mechanical.

High vacuum represents pressures between 10"3 and 10"6 ton absolute. Flow in this region is molecular or Newtonian, with very little interaction between individual molecules. A number of specialized industrial applications, such as ion implantation in the semiconductor industry, fall in this range. Nonmechanical ejector or cryogenic pumps (which are not discussed in this book) are usually used.

Very high vacuum represents absolute pressures below 10-6 torr. This isprimarily for laboratory applications and space simulation, Keep in mind that a "perfect" vacuum—that is, a space with no molecules or atoms—is a purely theoretical condition. Only in interstellar space is this condition approached at all closely, and even there a few atoms per cubic meter will be found. In practice, all vacuums are partial.